Current behavior:
Clients can only experiment on CSS on their own site. Any change they want to try out and test can impact their live site, meaning that if they make a mistake, or if their experiment isn't as good as expected it impacts the look and feel and usability of their public site.

Proposal:
Create a CSS sandbox area, either as part of help as as associated with a testing site, where they can try out different CSS setups and see how they look before doing them on their own site. This would allow them to practice CSS, as well as give them a place to experiment with different setups.